Emulsion blasting agents are well-known in the art. They are fluid when formed (and can be designed to remain fluid at temperatures of use) and are used in both packaged and bulk forms. They commonly are mixed with ammonium nitrate prills and/or ANFO to form a "heavy ANFO" product, having higher energy and, depending on the ratios of components, better water resistance than ANFO. Such emulsions normally are reduced in density by the addition of air voids in the form of hollow microspheres, other solid air entraining agents or gas bubbles, which materially sensitize the emulsion to detonation. A uniform, stable dispersion of the air entraining agent or gas bubbles is important to the detonation properties of the emulsion. Gas bubbles, if present, normally are produced by the reaction of chemical gassing agents. Sensitization also can be obtained by incorporating porous AN prills.
A problem associated with the use of emulsion blasting agents in mining blasting operations is the formation of nitrogen oxides, a yellow orange-colored smoke, in the gasses produced by the detonation of the emulsion blasting agent. These gasses will be referred to herein as "after-blast fumes." Not only is the formation of nitrogen oxides a problem from the standpoint that such fumes are toxic but also these fumes are visually and aesthetically undesirable due to their yellow/orange color. Many efforts have been made to eliminate or reduce the formation of such fumes. These efforts typically have been directed at improving the quality of the emulsion blasting agent and its ingredients to enhance the reactivity of the ingredients upon initiation. Other efforts have focused on improving blast pattern designs and initiation schemes. Still other efforts have focused on improving the borehole environment by dewatering or using a more water resistant emulsion blasting agent.
It surprisingly has been found in the present invention that the formation of nitrogen oxide fumes can be reduced considerably by adding urea, in an amount from about 5% to about 30%, by weight of the composition, to the oxidizer salt solution discontinuous phase of the emulsion or in dry form or both. The urea apparently reacts chemically with any nitrogen oxides that may form as products of the detonation reaction to convert such oxides to nitrogen (N.sub.2), water and carbon dioxide.
Additional advantages are realized by using urea to reduce nitrogen oxides in after-blast fumes. The use of urea in the oxidizer salt solution has been found to increase the critical diameter of the resulting emulsion blasting agent. Consequently, the emulsion blasting agent is more compatible (less reactive) with down-hole detonating cord that otherwise can cause a pre-detonation reaction to occur when the detonating cord is initiated. (The detonating cord leads to a booster located in the bottom of the borehole or a series of boosters spaced within the explosives column.) This pre-reaction itself can contribute to the formation of nitrogen oxides in after-blast fumes.
Another advantage is that the cost of using urea is considerably less than the costs of using plastic microballoons or sensitizing aluminum particles, which both have been used previously in an effort to improve the quality or reactivity of the emulsion blasting agent and its ingredients. Moreover, urea is more effective in chemically reducing nitrogen oxide after-blast fumes than these more costly alternatives.
By using urea, which is a fuel, in the oxidizer salt solution, less organic fuel can be used in the continuous organic fuel phase to achieve oxygen balance, particularly in emulsion blends containing ANFO or AN prills. This also appears to contribute to the reduction of after-blast nitrogen oxide fumes. Another advantage is that urea can extend or replace some or all of the water required in the oxidizer salt solution to result in a more energetic blasting agent.
Urea has been used or suggested for use in water-bearing blasting agents of the emulsion or water-gel type and in ANFO blasting agents. For example, U.S. Pat. No. 5,159,153 discloses the use of urea in the oxidizer salt solution phase of an emulsion blasting agent for purposes of stabilizing the blasting agent against thermal degradation in the presence of reactive sulfide and pyrite ores. U.S. Pat. No. 4,338,146 discloses the use of urea as an additive in a cap-sensitive emulsion explosive in an amount of less than 5% by weight. U.S. Pat. No. 4,500,369 discloses the use of urea in an emulsion blasting agent to lower its crystallization temperature. U.S. Pat. No. 3,708,356 discloses the use of urea to stabilize ANFO against reaction with pyrite ores. These patents do not suggest, however, the use of urea for the purposes described herein.